MB39A115 Datasheet, PDF (19/42 Pages) Fujitsu Component Limited. – 5 ch DC/DC Converter IC with Synchronous Rectification

MB39A115
■ SETTING THE SOFT-START TIME
To prevent rush currents when the IC is turned on, you can set a soft-start by connecting soft-start capacitors
(CS1 to CS5) to the CS1 terminal (pin 38) to CS5 terminal (pin 27) respectively.
As illustrated below, when each CTLX is set to “L” from “H”, ch.1 and ch.2 charge the soft-start capacitors (CS1
and CS2) externally connected to the CS1 and CS2 terminals at about 10 µA.
When each CTLX is set to “H” from “L”, ch.3 to ch.5 charge the soft-start capacitors (CS3 to CS5) externally
connected to the CS3 to CS5 terminals at about 1 µA.
The error amplifier output (FB1 to FB5 terminals) is determined by comparison between the lower voltage of the
two non-inverted input terminal voltage (1.23 V (ch.1 : 1.0 V) , CS terminal voltage) and the inverted input terminal
voltage (−INE1 to −INE5 terminal) . The FB terminal voltage is decided for the soft-start period (CS terminal
voltage < 1.23 V (ch.1 : 1.0 V) ) by the comparison between −INE terminal voltage and CS terminal voltage. The
DC/DC converter output voltage rises in proportion to the CS terminal voltage as the soft-start capacitor externally
connected to the CS terminal is charged. The soft-start time is obtained from the following formula :
Soft-start time : ts (time until output 100%)
ch.1
: ts (s) =: 0.100 × CSX (µF)
ch.2
: ts (s) =: 0.123 × CSX (µF)
ch.3 to ch.5 : ts (s) =: 1.23 × CSX (µF)
• Soft-start circuit (ch.1, ch.2)
Vo
R1
−INEX
VREF
10 µA
R2
CTLX
CSX
CSX
FBX
L priority
Error Amp X
−
++
1.0 V/1.23 V
X : Each channel number
19